A clutch means a device for connecting or disconnecting a pair of concentric rotary shafts in an easy and rapid way. The clutch is generally provided between shafts connected to an engine and a machine to operate or stop the machine, and also allows an engine to operate in a non-load state like a vehicle.
A clutch used in a commercial vehicle has a power transmission function for transmitting a rotating force of an engine to a transmission and a power blocking function for temporarily blocking a power flow between an engine and a transmission whenever required. The clutch also allows a machine to start operation softly without a vibration, protects the engine and the power transmission device against overloads, and reduces a rotation vibration of the engine together with a flywheel.
FIG. 1 shows a structure of a general clutch. As shown in FIG. 1 a clutch generally includes a clutch pedal 1 for receiving a manipulating force of a driver, a master cylinder 3 for transmitting the power of the clutch pedal 1, an air pump 5 connected to the master cylinder 3 to supply an air pressure, a booster cylinder 7 having a piston rod 7a operating with the air pressure supplied from the air pump 5, a release cylinder 9 for operating a clutch 11 with the air pressure provided from the booster cylinder 7, and a shift lever 13 hinged to connect the release cylinder 9 and the clutch 11. The push rod 9a of the release cylinder 9 has a restoring force by means of a spring 9b. 
In the clutch configured as above, if a driver steps down the clutch pedal 1, the force operates the piston rod 3a of the master cylinder 3 so that the air pressure supplied from the air pump 5 flows via an air pressure guiding unit 51 to the booster cylinder 7. The introduced air pressure moves the piston rod 7a of the booster cylinder 7, which allows the push rod 9a of the release cylinder 9 to move. If the push rod 9a moves, the shift lever 13 allows a clutch disc of the clutch 11 to be separated from a driving plate of an engine, thereby blocking the power transmitted to wheels.
However, when a vehicle is in a congested or slow-down section or drives in a city, the clutch pedal should be numerously operated, which increases the fatigue of a driver and strains the knee and the waste.
If an automatic transmission is mounted to solve the above convenience, even though a driver may feel convenience and comfort, an initial purchasing price increases, and fuel consumption increases by about 20 to 30% in comparison to a manual transmission.
To solve this problem, many semi-auto clutches have been proposed. However, a semi-auto clutch is complicated, which gives problems in repair and maintenance, and it is very difficult to attach the semi-auto clutch to an existing vehicle. Therefore, there is demanded a clutch control device which may be simply attached to or detached from an existing vehicle and ensure convenient maintenance.
In particular, if a semi-auto clutch is used, its rotation is read using a potentiometer by controlling a clutch operation with a motor, and the rotation is converted into a straight displacement to calculate a clutch clearance and control transmission. Due to this reason, the machine size increases and the semi-auto may not be easily mounted to a vehicle.
In addition, when converting the rotation into a displacement, an error is caused due to a mechanical SLIP or the like, which may result in unnecessary transmission impacts